1992 Rodeo

A couple of questions, can someone tell me where to find the diagnostic hookup on my 3.1 Rodeo? and secondly, Any suggestions why I'm getting TERRIBLE gas milage about 5 mpg city? Have a slight miss, I've checked the plugs and distributor

Your diagnostic hookup is right under the steering column.
On the other question - I don't know, sorry, but would like to know also (almost the same problem with me)